Golf balls generally have either a one-piece construction or several layers including an outer cover surrounding a core. A wound ball configuration typically has a vulcanized rubber thread wound under tension around a solid or semi-solid core, which is then enclosed in a single or multi-layer covering of tough, protective material. Another type of ball, a one-piece ball, typically formed from a solid mass of moldable resilient material which has been cured to develop the necessary degree of hardness. One-piece molded balls generally do not have an enclosing cover. Multi-piece (two or more pieces) non-wound balls generally have a solid or liquid core of one or more layers, and a cover having one or more layers formed over the core.
Many multi-piece golf balls have a cover containing an ionomer resin to impart toughness and cut resistance. Examples of such ionomers include Surlyn®, available from E.I. DuPont de Nemours and Company, and Iotek®, available from Exxon-Mobil.
Polyurethanes also have been used in cover materials of multi-piece golf balls.
Polyurethanes may be formed by mixing two primary ingredients during processing, most commonly a polyisocyanate, e.g., diphenylmethane diisocyanate monomer, toluene diisocyanate, or their derivatives, and a polyol, e.g., a polyester- or polyether polyol. An isocyanate that is reacted with a polyamine forms a polyurea. The term “polyurethane” is often used to describe polyurethane/polyurea systems. Polyurethanes may be thermoset, e.g., having a crosslinked molecular structure, or thermoplastic, e.g., having a linear molecular structure. A polyurethane becomes irreversibly “set” when a polyurethane prepolymer is crosslinked with a polyfunctional curing agent, such as a polyamine or polyol. The prepolymer typically is made from polyether or polyester. Crosslinking occurs between the isocyanate groups and the hydroxyl end-groups of the polyol. The physical properties of thermoset polyurethanes may be adjusted by the degree of crosslinking. For example, tightly crosslinked polyurethanes are fairly rigid and strong, whereas a lower level of crosslinking results in materials that are flexible and resilient. Depending upon the processing method, reaction rates may be quite fast, e.g., as in the case for some reaction injection molding (RIM) systems, or in other cases may be several hours or longer, e.g., as in several coating systems.